This invention relates to a process for refining of a crude sodium hexafluorosilicate containing gypsum as a principal impurity.
The most prevailing method for industrial preparation of sodium hexaflurosilicate (sodium silicofluoride) is the neutralization of hexafluorosilicic acid obtained as a by-product of the preparation of wet process phosphoric acid with a sodium compound such as sodium hydroxide, sodium carbonate or sodium chloride.
It is no exaggeration to say that sodium hexafluorosilicate obtained by this method always contains a substantial amount of calcium sulfate dihydrate or gypsum as a principal impurity due to the presence of gypsum either dissolved or suspended in a phosphoric acid solution used as the starting material. It is practically impossible to completely remove solid phase gypsum from either a phosphoric acid solution or a hexafluorosilicic acid solution by means of an industrial filter. Even if an ideal filtration were realized, crystallization of gypsum from the filtrate at the stage of precipitation of sodium hexafluorosilicate by the neutralization is inevitable. In industrial preparation of sodium hexafluorosilicate from wet process phosphoric acid, therefore, it is next to impossible to preclude the presence of gypsum in the product.
In addition, sodium hexafluorosilicate of this origin usually contains impurity metal elements such as iron and aluminum in the form of insoluble and sparingly soluble phosphates since these elements are usually dissolved in the starting phosphoric acid solution.
In recent years increasing attention has been focused on sodium hexafluorosilicate as an industrially important fluorine source with development of practicable processes for the preparation of important fluorides exemplified by synthetic cryolite and aluminum fluoride from the fluorosilicate, and accordingly there is an eager demand for industrial supply of refined sodium hexafluorosilicate of which impurity content is small enough to give an intended fluoride with satisfactorily high purity. Since prevention of the inclusion of gypsum in sodium hexafluorosilicate is practically impossible, the demand must be met by refining of an industrially obtained crude sodium hexafluorosilicate.
U.S. Pat. No. 3,055,733 discloses that crude sodium hexafluorosilicate of 94-97% purity can be purified to the extent of 98-99% by repulping at 80.degree. C. using phosphoric acid (of 30% P.sub.2 O.sub.5) and 13% hexafluorosilicic acid solution each in the same weight as the crude fluorosilicate. However, this method will be disadvantageous from the economical viewpoint because of the consumption of costly phosphoric acid and hexafluorosilicic acid in large amounts. Besides, the repulping at a temperature as high as 80.degree. C. may present a problem about the material of the repulping apparatus. U.S. Pat. No. 3,915,659 discloses a refining process in which crude sodium hexaluorosilicate is repulped with a sodium chloride solution to selectively remove gypsum based on a difference in solubility. This process is easy to practise but has the disadvantage of requiring the disposal of an immense quantity of waste water resulting from the use of the sodium chloride solution in a very large quantity, accompanied by a considerable loss of sodium hexafluorosilicate during the treatment.